1. Field of the Invention
The present invention relates to an optical fiber grating and to a manufacturing method therefor. More particularly, it relates to an optical fiber grating which is low cost and exhibits small changes over time, and to a manufacturing method therefor. The present specification is based on Japanese Patent Application No. Hei 9-247292, filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
An optical waveguide grating has a spatially periodic perturbation of waveguide structure, formed in a longitudinal direction of an optical fiber or of a Planar Lightwave Circuit (PLC).
This optical wageguide grating is a device which is capable of making loss of light in a specified wavelength band, by generating couplings between the mutual specified modes. Then, having such a characteristic, it may be utilized as coupling-type devices for the elimination of light for specified wavelength band and as coupling-type devices between specified modes.
The optical waveguide grating may be classified into a reflecting type and a radiating type, according to the relationship between the coupling modes.
Here, let the direction of incidence of light for the optical waveguide be the positive direction, and a direction opposite to it, the negative direction.
A reflecting-type optical waveguide grating has been imparted with a characteristic for reflecting light of specified wavelength, by coupling a mode which propagates through the core in the positive direction and a mode which propagates through the core in the negative direction.
A radiating-type optical waveguide grating has been imparted with a characteristic for radiating by coupling a mode which propagates through the core and a mode which propagates through the cladding, so as to obtain the characteristic by having the light of specified wavelength which is radiated to the outside of the waveguide and then attenuated.
Now, the periodic perturbation in the waveguide structure for an optical waveguide grating may be formed by changing the index of refraction for the core or the core diameter.
The most general method of manufacturing an optical waveguide grating is one in which the index of refraction of the core is made to change by a phtotorefractive effect (sometimes also called the xe2x80x9cphotosensitive effectxe2x80x9d).
The photorefractive effect is a phenomenon whereby, for example, when silica glass with germanium as the dopant is irradiated with ultraviolet rays in the neighborhood of wavelength 240 nm, an increase in the index of refraction is observed for the silica glass.
Specific description of the optical fiber will be given concretely as an example as follows.
FIG. 14 is a diagrammatic configuration drawing which describes the manufacturing process for a conventional optical fiber grating.
In the drawing, reference symbol 11 represents an optical fiber, and this optical fiber 11 is composed of the central part thereof, core 11a, and a cladding 11b which is provided on the outer circumference of this core 11a. 
This optical fiber 11 is, for example, an optical fiber which act as a single mode at a wavelength of 1.55 xcexcm (the xe2x80x9csingle-mode optical fiberxe2x80x9d).
The core 11a is made of silica glass added germanium as a dopant. Germanium is normally added as germanium dioxide to the silica glass.
In this example, the core 11a is made of silica glass containing 5 wt % germanium dioxide, and the cladding 11b is composed of silica glass the purity of which is effectively at about a level in which the dopant can be ignored (hereinafter, referred to as xe2x80x9cthe pure silica glassxe2x80x9d).
Hereinafter, pure silica glass or silica glass added with a dopant, may at times be referred to as the xe2x80x9cthe silica based glassxe2x80x9d.
The reference symbol 12 is a phase mask, and this phase mask 12 is made of silica glass. A plurality of gratings 12a is formed at specified intervals on the one side.
The grating part 13 may be formed in the following manner: namely, an ultraviolet laser beam of wavelength 240 nm from an ultraviolet laser generator (not shown) is irradiated on the side surface of the optical fiber 11 via the phase mask 12.
As ultraviolet ray laser generator, KrF excimer laser and the like is used.
Whereupon, an interference fringe is generated from plus first-order diffracted light and minus firstxe2x80x94order by the gratings 12a of the phase mask 12 from the irradiation of the ultraviolet ray laser beam. Then, the index of refraction for the part of the core 11a in which this interference fringe has been generated changes, and as a result, the relative refractive index-difference between the core 11a and the cladding 11b changes.
In this manner, a periodic change in the index of refraction for the core 11a is formed along the longitudinal direction of the optical fiber 11. Then, a grating part 13 which is formed with a periodic change for the relative refractive-index difference between the core 11a and the cladding 11b is obtained.
Here, what determines the characteristic as to the optical fiber grating being either a radiating type or a reflecting type is the grating period, representing the period of the change in the index of refraction of the core 11a (the period of the relative refractive index-difference between the core 11a and the cladding 11b).
Now, assume that the propagation constant of the mode, which propagates in the optical fiber is xcex2 1, and the propagation constant of the mode to be coupled, is xcex2 2. Then the difference in the propagation constants between these coupling modes, xcex94 xcex2, is represented by the following equation (1):
xcex94xcex2=xcex21xe2x88x92xcex22xe2x80x83xe2x80x83Equation (1). 
Now, grating period xcex9 is given by the following equation (2), where:
xcex9=2xcfx80/xcex94xcex2xe2x80x83xe2x80x83Equation (2). 
Here, the propagation constants xcex2 1 and xcex2 2 for light are taken as being positive in the direction of the incidence of light and negative in the direction opposite to that of the direction of incidence.
The approximate values taken by xcex2 1 and xcex2 2 are roughly equal to 2 xcfx80 divided by the wavelength of light propagating in the optical fiber. The orders of magnitude of the values are roughly equal to the wavelength of light in a vacuum divided by the index of refraction of the optical fiber.
For example, the orders of magnitude of the various values as a guide are set as follows:
Wavelength of light (in vacuum): 1.55 xcexcm.
Index of refraction of optical fiber:
approximately 1.5
Wavelength of light in optical fiber (wavelength in the guide): approximately 1 xcexcm. xcex21 and xcex22: approximately 2 xcfx80 rad/xcexcm.
When the grating period xcex9 is short, the optical fiber acts as a reflecting type, and when the grating period is long, the optical fiber grating acts as a radiating type.
For this reason, there are cases in which the reflecting-type optical fiber grating will be called the xe2x80x9cshort-period optical fiber gratingsxe2x80x9d, and the radiating-type optical fiber grating will be called the xe2x80x9clong-period optical fiber gratingxe2x80x9d.
For example, when the grating period xcex9 is 0.5 xcexcm, the optical fiber grating used is the reflecting type. That is, a certain mode of light incident from one end of this optical fiber grating (optical fiber 11) couples with the other mode, which proceeds in the core 11a in a direction opposite to that of the incident light (the negative direction) and turns into a reflected light.
This reflected light suffers a loss in the outgoing light, so that it may be used as a device for imparting a loss in a specified wavelength band.
The value of the grating period xcex9 of 0.5 xcexcm, at this time, corresponds to approximately one half of the wavelength of light in the optical fiber (wavelength in the guide) which has been indicated as the aforementioned guide. By imparting disturbances of such a short period in the longitudinal direction of the optical fiber 11, an indication is made that the light is made to be reflected in the opposite direction.
In contrast to this, the radiating-type optical fiber grating is one for which the grating period xcex9 of Equation (2) is long. When the optical fiber grating is of the radiating type, the grating period xcex9 is usually from several tens to several hundreds of xcexcm.
The fact that the grating period xcex9 is long, indicates that the difference in the propagation constants between modes, xcex9xcex2 which is involved in the coupling is small, and that couplings between two modes which propagate in the same direction can be generated.
The mode, which has a light incident onto this optical fiber grating coupling with the cladding mode, is radiated as a radiating light to the outside of the core and is attenuated. The light of the mode thus radiated suffers a loss in an outgoing light, so that this optical fiber grating may be used as a device for imparting a loss in the specified wavelength band.
An example of this reflecting-type optical fiber grating has been disclosed in Japanese Patent Application, First Publication No. Hei 7-283786.
In this Publication, disclosure has been made for a radiating-type optical fiber grating, whereby a laser beam is irradiated with a KrF laser through an amplitude mask onto an optical fiber having a core made of silica glass doped with germanium, and the optical fiber grating is formed in which the change in the index of refraction of the grating with a period in a range of 50 to 1500 xcexcm has been formed in the core of the optical fiber.
However, the following problems existed in the conventional art optical fiber grating.
Namely, the combinations of the wavelengths of the light source and the dopant which is added to the optical fiber are restricted. Accordingly, the kinds of light source are restricted as well.
Realistically speaking, in the manufacture of optical fiber gratings, in order to take advantage of the photorefractive effect, the optical fibers are restricted to those to which germanium has been added to the core, and the wavelengths which are capable of generating photorefractive effect in the silica glass doped with germanium has been added are restricted to the wavelengths in the neighborhood of 240nm.
As laser generators capable of irradiating ultraviolet laser beam of such wavelengths, KrF excimer laser, the second harmonic of the argon laser with a band at 480 nm and the like are included. However, each of these is expensive and is a factor contributing to the increase in the manufacturing cost.
In addition, the change in the index of refraction for the silica glass doped with germanium, obtained by once irradiating ultraviolet ray laser beam from KrF excimer laser and the like, is of the order of 10xe2x88x924 to 10xe2x88x923 and is not so large. For this reason, in order to obtain a relatively large change in the index of refraction, it is necessary that a given spot be irradiated a large number of times with an ultraviolet ray laser beam; thus the manufacturing process becomes long.
Moreover, since the change in the index of refraction of the optical fiber due to the photorefractive effect is based on the structural defects of silica glass, the stability is not sufficient.
Specifically, the change in the index of refraction caused to be generated in the silica glass doped with germanium, exhibits a conspicuous change within several hours, under the condition of high temperature environment of 200xc2x0 C. or higher. Also it has been known that a substantial reduction of the change in the index of refraction takes place at temperatures exceeding 300xc2x0 C.
It is an object of the present invention to provide an optical fiber grating which has high manufacturing efficiency, without requiring expensive equipment, and to provide a manufacturing method.
It is another object of the present invention to provide an optical fiber grating which exhibits stable grating characteristics over time and to provide a manufacturing method therefor.
In order to achieve these objects, in the present invention, a preform which has softening temperature of its core is heated to a temperature higher than the softening temperature of the cladding. The optical fiber provided with a core, having residual stress which has been obtained by drawing, is intermittently heated, the cladding in the periphery of this core is softened, and the index of refraction of the core is changed from releasing the residual stress, so as to obtain the optical fiber grating by forming a periodic change in the relative refractive index-difference of the core and the cladding in the longitudinal direction of the optical fiber.
For the present invention, the following effects are obtained.
Namely, since the heating means for forming the grating part is not restricted to the wavelength of the laser beam concerned, instead of expensive equipment such as excimer lasers, relatively inexpensive carbon dioxide gas lasers and the like may be used.
Furthermore, since the laser power required for softening the cladding is relatively small, the stress of the core is released so that the index of refraction can be increased, even though the number of scans traversing the optical fiber may be small.
Consequently, the cost of the manufacturing equipment is low, the manufacturing time is short, the operation is simple, and manufacturing efficiency is superior. For these reasons, reduction in cost can be obtained.
Since the periodic changes for the index of refraction of the core (spatially periodic changes for the relative refractive index-difference between the core and the cladding) of this optical fiber grating is of structural nature, there is little change with respect to the passage of time, an optical fiber grating which is stable over a long period of time is obtained.
The optical fiber grating of the present invention is capable of flattening the wavelength dependence of optical devices, for example such as light source, photodetector, light amplifier, optical fiber and the like which have wavelength dependence in the gain- wavelength characteristic, so that it can be used in various optical communication systems.